Conventional digital logic circuits require a stable direct current (DC) power, which may be supplied by a DC power source, such as a battery, or from an alternating current (AC) power supply that is rectified to produce DC power to supply the circuit. Devices such as radio frequency identification circuits (RFIDs) are typically very small devices that are powered by the radio frequency (RF) signals used to communicate with the device. For example, an RFID device may use RF signal received by the communication antenna as the input into a diode bridge rectifier, where the output of the rectifier is the DC power supply for the logic circuits of the RFID device. The use of ambient AC signals as a source of power is commonly referred to as “power harvesting.”
One disadvantage to conventional devices that employ power harvesting to power logic circuits, however, is that the rectifier circuit can be bulky relative to the digital logic circuit. Another disadvantage is that, because bridge rectifiers use diodes, the peak-to-peak amplitude of the RF signal output from the antenna must be more than twice the diode drop, e.g., VRF>2×VDIODE. For most technologies, VDIODE is 0.5V or greater. Thus, the usefulness of conventional RF to DC converters is limited to about 1V RMS.
As a result, conventional RF powered digital circuits have several disadvantages, such as that they only work at low frequencies and require the recovered RF supply voltage to be greater than the threshold voltage, which limits the range (i.e., the distance from the source of the RF signal) at which the RF powered circuit can operate. Another disadvantage is that conventional RF powered digital circuits require power-on-reset circuits to save their results to DRAM, which hold the stored bit during the circuits' off time. Yet another disadvantage is that in an RF powered application, the recovered voltage is limited by the size of the antenna, the number of turns in the loop, the quality of the matching network, the complexity of the rectification circuit, and the distance and orientation to the RF source. Thus, there is a correlation between the size of the RFID device and its operating range. A smaller RFID is desired, but reducing the size of the RFID device reduces the range, which is undesired. Any reduction in the required AC operating voltage would increase the operating range of a RFID device without an increase in size of the RFID device, which is a significant benefit.
It would therefore be desirable to have AC powered logic devices that operate at lower field strengths with lower requirements on RF voltage, especially in applications driven by RF power harvesting. Accordingly, in light of these disadvantages associated with conventional power harvesting in general and with RF powered digital circuits in particular, there exists a need for AC powered logic circuits and systems including same.